The most common material used for bearings is bearing steel. There are extremely stringent requirements regarding the internal structure of the bearing, the uniformity of its chemical composition, the presence of non-metallic inclusions, the size and distribution of carbide particles, and the degree of decarburisation; otherwise, these defects will significantly reduce the service life of the bearing and the suitability of the material for use.
1. Purity of Bearing Steel: Fatigue spalling of bearing components is related to non-metallic inclusions in the steel, with oxides having a particularly significant impact. This is because, when rolling bearings are in rotation, the contact surfaces of the bearing components are subjected to pulsating loads; non-metallic inclusions cause localised disruption to the continuity of the steel matrix, leading to internal stress concentration, plastic deformation around the inclusions, and the formation of microcracks.
Furthermore, the content of gases (oxygen, nitrogen and hydrogen) in bearing steel is also an important indicator of purity. As the temperature decreases, oxygen dissolved in the steel precipitates and forms oxide inclusions with elements such as aluminium, calcium, silicon and manganese; whilst nitrogen forms highly dispersed aluminium nitride inclusions and coarser titanium nitride and titanium carbonitride inclusions in the steel. When accumulated hydrogen atoms combine to form hydrogen molecules, they generate immense pressure; once this exceeds the steel’s strength limit, internal cracking occurs, resulting in white spots. Reducing gas content is a crucial step in improving the purity of bearing steel.
In recent years, most major bearing steel producers in China have successively commissioned ultra-high-power electric arc furnaces. By employing advanced eccentric furnace-bottom tapping technology and carrying out secondary refining, they have reduced the oxygen content in the steel to around 10×10⁻⁶, with some achieving a minimum oxygen content of (4–3)×10⁻⁶. The secondary refining ratio has also reached 100 per cent. Consequently, the quantity of inclusions in the steel has been effectively controlled.
2. Homogeneity of Bearing Steel
The uniformity of bearing steel refers to the uniformity of both chemical composition and carbide distribution. Factors such as ingot structure, ingot weight, casting temperature and casting method influence the distribution of chemical composition within the steel. The heating process prior to hot working of ingots and billets, the final temperature of hot working, the subsequent cooling method and the spheroidising annealing process all affect the uniformity of carbide distribution. Based on the shape, distribution and causes of carbide non-uniformity in the microstructure of bearing steel, carbides can be classified as liquid-precipitated carbides, banded carbides and network carbides. The harmful effects of liquid-precipitated carbides are comparable to those of inclusions in steel; banded carbides rated at grades 3–4 can reduce the fatigue life of the steel by 30 per cent. A one-grade increase in reticulated carbides can reduce bearing life by one-third, whilst carbide particle size directly or indirectly affects bearing life. The grading of liquid-segregated carbides, banded carbides and reticulated carbides serves as an indicator of carbide uniformity.
3. Dimensional Accuracy and Surface Quality of Steel
Improving the dimensional accuracy of steel not only enhances the utilisation rate of bearing materials but also reduces unnecessary machining. Some steelworks employ high-rigidity rolling mills with short stress lines, resulting in a significant reduction in dimensional deviations of the steel cross-section; for round bars with a diameter of 30 mm or less, the diameter deviation can be as low as approximately 0.3 mm.
The service life and reliability of bearings are related to factors such as structural design, materials used, hot and cold working processes and equipment, as well as installation and operating conditions. Among these, materials and heat treatment are among the primary factors influencing bearing service life and reliability. Due to the influence of materials and heat treatment processes, factors such as low material strength, poor purity, inappropriate orientation of the metal’s fibrous structure, low hardness, unsuitable forging ratios and high residual stresses can very easily lead to bearing failure. For example, Xingcheng Steel and Northeast Special Steel, which are used by Kent Bearings, are among the better domestic manufacturers of bearing steel. Their bearing steel offers greater wear resistance, resulting in bearings with a longer service life.
